1. Field of the Invention
The invention relates to zinc oxide varistors and somewhat more particularly to a method of producing ceramic materials for such zinc oxide varistors.
2. Prior Art
Varistors are resistors with non-linear current-voltage characteristics. As soon as an electrical voltage connected to a varistor exceeds a specific critical value, the electrical resistance value of such varistor strongly decreases. The current-voltage behavior of a varistor is typically expressed by the equation I=(U/C).alpha., wherein U is the voltage connected to the varistor, I is the current flowing through the varistor, C is a proportionally constant and the exponent .alpha. is the non-linear coefficient.
Varistors are increasingly utilized for over-voltage limitations, as protection against power surges and also for voltage stabilization. Older developments provided varistors composed of silicon carbide materials having .alpha.-values of approximately 3 to 7. More recent developments provide metal oxide varistors, such as for example, composed of zinc oxide materials. With zinc oxide varistors, for example, the varistor effect occurs at considerably lower voltages than with the earlier silicon carbide varistors so that zinc oxide varistors can also be utilized in automotive electronics. Typically, zinc oxide varistors have .alpha.-values ranging between 15 and 60.
In order to stabilize the varistor effect, particularly the .alpha.-value at low current density, in addition to a more or less large number of other additives, antimony has been added to zinc oxide starting materials. Antimony produces a stabilizing effect for the electrical properties of zinc oxide materials utilized for varistors.
J. Appl. Physics, Vol. 46 (1975) pages 1653-1659 describes investigations relating to microstructure and phase transformation in metal oxide varistor ceramics. According to these investigations, antimony present in the structure of a zinc oxide varistor, is, for example, in a phase thereof corresponding to a Zn.sub.7 Sb.sub.2 O.sub.12 spinel in which antimony is in its pentavalent form.
German Offenlegungsschrift No. 27 52 150 (which generally corresponds to U.S. Pat. No. 4,169,071) suggests adding antimony to a zinc oxide starting material in the form of the aforesaid Zn.sub.7 Sb.sub.12 O.sub.12 spinel, preferably in a defined amount and grain size.
It has been determined that it is favorable for promoting large grain growth in the zinc oxide material to add antimony in its pentavalent form, rather than in its trivalent form. More coursely grained zinc oxide causes varistors containing such course-grained zinc oxide to have relatively low threshold voltage for the varistor effect. It has been shown that it is relatively difficult to produce Zn.sub.7 Sb.sub.2 O.sub.12 relative to purity of pentavalent antimony, so that residual components of trivalent antimony (Sb.sub.2 O.sub.3) (which are disadvantageous for desired properties in zinc oxide materials for varistors materials) are no longer contained therein.